Frequency-modulation receiving system



Oct. 14, 1941. w. L. cALSON 1 2,258,599

FREQUENCY-MODULATION RECEIVING SYSTEM Filed June 29, 1940 nvenlor vHomey Patented Oct. 14, 1941 FREQUENCY-MODULATION RECEIVING SYSTEMWendell L. Carlson, Haddoniield, N. J., assigner to Radio Corporation ofAmerica, a corporation of Delaware Application June 29, 1940, Serial No.343,109

(Cl. Z50-20) 7 Claims.

ceiving system having a frequency-discriminator network adapted for thereception and demodulation of amplitude modulated signals.

A further object of the present invention is to provide an improvedfrequency-discriminator network for the reception and conversion ordemodulation of ultra high-frequency amplitude and frequency-modulatedsignals selectively, and

lof relatively low frequency amplitude-modulated signals, with a minmiumof circuit changes.

It is a further object of the present invention to provide an improvedfrequency-discriminator network for the reception of both amplitude andfrequency-modulated signals and for tuning indication, with a minimum ofvcircuit changes;

The invention will further be understood from the following description,when considered in connection with the accompanying drawing, and itsscope is pointed out in the appended claims.

In the drawing,

Figure 1 is a schematic circuit diagram of a combined amplitude andfrequency-modulation receiving system embodying the invention;

Figure 2 is a similar schematic circuit diagram of a modification of aportion oi the circuit of Fig. 1; and

Figure 3 shows curves illustrating certain oper- `ating characteristicsof the circuit of Fig. 1.

Referring to Fig. 1, the circuit 5 is a frequency discriminator networkfor the reception of frequency modulated signals, provided with a pairof balanced rectiers 6 and 1 preferably of the diode type as shown, fromwhich ,converted signals are applied through suitable volume controlmeans 8 to an audio frequency ampliiier system 9 and output device orloudspeaker I8.

The discriminator network is preceded by suitable intermediate frequencyamplier means Arepresented by the amplifier stage comprising the tube II and the input portion of a multiple band receiving system indicated atI2, through which signals are selected from any suitable signal sourcesuch as an antenna circuit I3.

The receiving system is of the multiple wave band type for receivingsignals in a plurality of differing wave bands, such as an ultra highfrequency amplitude and frequency modulated signal band and the normalbroadcast and short wave bands at present in use for entertainmentprograms. The wave band change means is indicated at I4, and variabletuning means for each band is indicated at I5. As such wave band tuningand selecting systems are known and as any suitable system may beprovided at I2, further description is unnecessary.

The discriminator network 5 includes a tuned secondary inductance I6having terminals connected with the diode rectifier anodes I1 and I8 andhaving an electrical center tap I9 connected to a similar center tap20on the balanced rectifier output impedance or resistor network 2|22,each half of which is provided with a suitable bypass capacitor 23. Thediode cathodes 24-25 are connected with the terminals of the impedance2I--22, the cathode 25 preferably being grounded as indicated at 26. Thecathode 24 is connected to a switch 21 having a contact 28 providing aconnection with the ungrounded terminal of the output impedance andhaving a second contact 29 connected to the grounded or rst named outputimpedance terminal preferably through a ground connection as indicatedat 30.

The volume control device 8 is connected in shunt with the outputimpedance 2 I--22 through a coupling capacitor 3l and is preferably ofrelatively high resistance, thereby to provide a relatively light loadon the rectifier circuit and output impedance.

In accordance `with the invention theuse of a choke coil between thesecondary terminal I9 and the terminal 20 on the output 'impedance isobviated by interposing in the circuit ordinarily occupied by the chokecoil one or more serially connected intermediate secondary windings suchas a high frequency winding 35 and a low frequency winding 36, inseries, the lower frequency winding being more adjacent to the terminal28.

The main secondary I6 and the intermediate secondary 35 are each tunedto the same high intermediate frequency, such as 5 mc., for example, bysuitable shunt capacitors 3l and 38, respectively, the exact tuningadjustment being ellected preferably by means of movable ferromagnetictuning cores indicated at 39 and 40.

The secondary winding 35 is coupled to a primary winding 4I which isconnected in the output circuit 42 of the amplifier stage II and iscontacts 2 9Aand Vis; completed;

tuned by a shunt capacitor 43 to the same intermediate frequency of 5mc., the final tuning adjustment also being effected preferably by asuitable ferro-magnetic tuning core indicated at 44.

The tuned secondary 36, for low frequency broadcast and short wavesignals, for example, is tuned to a low intermediate frequency such as455 kc., by a suitable shunt capacitor indicated at 45 and is providedwith a primary 46 coupled thereto as indicated and tuned by a similarshunt capacitor 41. Both primary and secondary are provided withsuitable ferro-magnetic tuning cores indicated at 48 for the purpose ofexact tun-I ing to the desired intermediate frequency.

With this arrangement, the tuned primary or primaries in thediscriminator'network provide coupling between theintermediate'frequency output circuit and the balanced or push pullsecondary I6 and one or more additional secondary windings 35 and 36which are connected-,through the secondary I6 of the diode rectiers 6and 'I in parallel, that is, serially between 'substantially the"electrical centers 'of' the' main secondary I6 Vand of the rectifieroutput impedance 2 I-22. 'In' the present example, the connectionbetween the lowest frequency intermediate' secondary and the'outputresistor or impedance .center tap is provided by a lead indicated at 50-fr'om' which A. V. C. potentials may be derived with' respect to groundthrough a lead 5I for the input portion of the receiving system' asindicated, in which case the I.F. amplifier I I is preferably operatedwithout appreciable signal limiter characteristics-` Y `With thisarrangement, whether operating in response to'amplitude orfrequency-modulated signals, the A. V. C'. potential is derived from the're'si'stor section 22 of therectifler output'circuit is provided inwhich the rectiiiers are connected with themain secondary I6 Aand theoutput impedance 2I-22 in balanced or push-pull relation thereby toprovide voltages in phase quadrature at resonance and to convert thefrequency modulation on'V the carrier to audio signals in the In orderto receive amplitude-modulated signals 'at 'either the highYintermediate frequency of 5 mc., or the low intermediate frequency of455 kc., switch 21 is moved to the contact 29 vthereby placing thedioderectiiiers in parallel -withrespect to either of the tunedsecondaries -35'and 36 with the cathodes connected to ground.

4-The impedance of the low induc'tance, high frequency secondary I6 incircuit with the secondaries 35 and 36 and the impedance 'of' therelatively low Ainductance 'high' frequency secondary 35 in circuit withYthe low freequency high "indu'ctance primary 36 is substantiallynegligible.

'For' tuningv indication,` either for amplitudemodulated orAfrequency-modulated signals, a

4timing indicator device indicated at 55, may be 'connectedwith thelead, 50 through a switch 56 `hai/'fing a contact`5l'connected with thetuning' 'dev ic'and thence to ground,` as indicated at 58,

wherebythe tuning indicator is placed in parallel -with the outputresistor section 22. Y

' :Forffrequencymodulation reception, it is desirable to provide' tuningindication during the- Ytuning operation. In'the present system this isprovided by closing the switches 2'I and 56 to the 51, respectively,until the tuning Where a circuit of the Conrad type is used, thediscriminator network may be modified as shown in Fig. 2 to whichattention is now directed, and in which like reference numerals refer tothe same circuit elements as in Fig. 1.

Referring to Fig. 2, the intermediate frequency amplifier stage II isprovided with an output circuit 42 including a pair of series connectedprimary windings 60 and 6I and a third primary winding 62 to the lowintermediate frequency by a shunt capacitor 63 together with aferro-magnetic tuning core 64.

The primaries 6I) and 6I are coupled to secondaries 65 and 66,respectively, one being tuned above and the other being tuned below themidfrequency or carrier, of the frequency modulated signal, by suitableshunt capacitors 6'I and 68, respectively, together with ferro-magnetictuning cores 'II and l2. The rectiiiers 6 and 'I are connected with themistuned input circuits and are differentially coupled to the outputimpedance 2 I-22 as in the preceding modification.

In the present circuit, however, the center tap for the input circuit islocated between the secondaries 65 and 66, as indicated at 13, and thelow frequency secondary 'I4 is included serially in circuit between thecenter tap I3 and the center tap 20 on the output impedance, through thelead 50. In the present example, the switch 56 connected with the lead50 is arranged to connect the tuning indicator circuit and A. V. C. lead5I simultaneously to the lead 50 through the contactV 5'I for amplitudemodulation reception only, and tuning indication with amplitudemodulation reception, the switch 2I being moved to the contact 29 toconnect the rectifiers in parallel. Therefore, these switches may bearranged to be operated jointlyas indicated by the dotted connection'15.

`As in the preceding embodiment, the secondary 'I4 of the lowintermediate frequencyV transformer is tuned by a capacitor I6 inconjunction with the variable tuning core indicated at TI.

The invention is applicable to any frequency discriminator networkwherein the series secondary circuit may be connected with a balancedsecondary circuit to provide push-pull or balanced operation of therectifiers for frequencymodulation reception and parallel operation foramplitude-modulation reception. This circuit arrangement furthermorepermits the use of 'a rectifier output impedance and simplifiedswitching means for connecting the diode output electrodes to theterminals of the output impedance in balanced relation and to the sameterminal of said impedance for parallel operation.

The operation of the tuning indicator system in response to frequencymodulation and amplitude modulation signals will be readily understoodby reference to the curves of Fig. 3, in which the curve86-'8I shows aresonance dip at 82 showing that the sound output for frequency 'modula--tion signals falls olf at resonance, while the tunattesta that thetuning indicator may be used for frequency modulation tuning indicationproviding silent tuning at the resonance point which is a desirablefeature of the tuning indication system shown and described.

It is obvious that the switching control for the tuning indicator anddetector may be incorporated with the tuning system in any suitablemanner other than that shown and described herein by way of example.

I claim as my invention:

1. A radio signal receiving system comprising in combination, tunablesignal receiving and selecting means, a frequency discriminator networkfor multiple wave band reception coupled to said first-named means,rectifier means in said network including a pair of rectifier elements,means for connecting said elements in balanced relation to each otherfor frequency-modulation reception in one wave band, and means forconnecting said elements in parallel relation to each other through saidlast named means for amplitudemodulation reception in another wave band,and tuning indication means connected to said rectiers in said parallelrelation for tuning said system to frequency-modulated signals.

2. A radio signal receiving system comprising in combination, tunablesignal receiving and selecting means, a frequency discriminator networkfor multiple wave band reception coupled to said first-named means,rectifier means in said network including a pair of rectifier elements,means for selectively connecting said elements in balanced relation toeach other for frequency-modulation reception in one wave band and inparallel relation to each other for amplitude-modulation reception inanother wave band, and means for producing a peak signal indication fortuning to resonance with a frequency modulated signal with substantiallyZero output.V Y

3. A radio signal receiving system comprising incombination, amplifyingmeans for selected signals having an output circuit, a frequencydiscriminator network including a plurality of primary windingsconnected with said last-named circuit, a plurality of secondarywindings coupled to said primary windings, rectifier means connected inbalanced relation to one of said secondary windings, means providing acircuit through said one winding and others of said secondary windingsin series for operating said rectifier means in parallel, an outputcircuit for said network including output impedance means, and means forconnecting said rectifier means selectively in balanced rel-ation and inparallel relation to each other through said output impedance means.

1l. A radio signal receiving system comprising in combination,amplifying means for selected signals having an output circuit, afrequency discrimin-ator network including a plurality of primarywindings connected with said last-named circuit, a plurality ofsecondary windings coupled to said primary windings, rectifier meansconnected in balanced relation to one of said secondary windings, meansproviding a circuit through said one winding and others of saidsecondary windings in series for operating said rectiiier means inparallel, an output circuit for said network including output impedancemeans, means for connecting said rectiner means selectively in balancedrelation and in parallel relation to eaolh other through said outputimpedance means, and means for deriving a tuning indication from saidoutput circuit when said rectifier means are connected in said parallelrelation to each other.

5. In a multiple waveband signal receiving system, the combination of afrequency discriminatory network comprising a plurality of primarywindings, a plurality of secondary windings coupled to said primarywindings, and tuned one to a relatively high intermediate frequency andanother to a relatively low intermediate frequency, rectifier meansconnected in .balanced relation to one of said secondary windings, acircuit through said one winding and others of said secondary windingsin series for operating said rectiiier means in parallel, an outputcircuit for said network including output impedance means, means forconnecting said rectifier means selectively in balanced relation and inparallel relation to each other through said output impedance means, andmeans coupled to said output impedance for deriving a converted signaltherefrom.

6.111 a frequency-modulation receiving system, a frequency discriminatornetwork comprising a plurality of tuned inductances, rectifier meanscomprising a pair of rectifier elements, a rectier output circuitincluding an output impedance, a circuit connecting said rectifierelements in balanced relation between said inductances and said outputimpedance, said circuit including at least two of said inductances inseries, means for tuning one of said seriesconnected inductances tosignals one at a relatively low frequency and another at a relativelyhigh frequency, and selective switching means for connecting saidrectifier elements in parallel relation with each other to said outputimpedance for response to signals at one of said frequencies.

7. In a frequency-modulation receiving system, a frequency discriminatornetwork comprising a plurality of tuned inductances, rectifier meanscomprising a pair of rectifier elements, a rectier output circuitincludingWan output impedance, a circuit connecting said rectifierelements in balanced relation between said inductances and said outputimpedance, said circuit including at least two of said inductances inseries, means for tuning one of said series-connected inductances tosignals one at a relatively low frequency and another at a differing`frequency, selective switching means for connecting said rectifierelements in parallel relation with each other to said output impedancefor response to signals at one of said frequencies, and means forderiving a tuning indication from said output impedance in response toparallel operation of said rectifier elements.

WENDELL L. CARLSON

